Why Does My Recursive Python Function Return None Instead of the Expected Path?

Recursive Functions and Returning None in Python [Duplicate]

In Python, recursive functions can be an effective tool for solving complex problems. However, it's important to understand the mechanics of recursion to avoid unexpected results.

Consider the following code snippet:

def get_path(dictionary, rqfile, prefix=[]):
    for filename in dictionary.keys():
        path = prefix + [filename]
        if not isinstance(dictionary[filename], dict):
            if rqfile in str(os.path.join(*path)):
                return str(os.path.join(*path))
        else:
            get_path(directory[filename], rqfile, path)

This function aims to return the path to a specific file within a nested dictionary. When executed, however, the issue arises that it returns None instead of the expected path.

The problem lies in the handling of the recursive call. To correctly implement recursion, the result of the recursive call needs to be returned to the calling function. In this case, it should be done within the else block:

else:
    return get_path(directory[filename], rqfile, path)

This ensures that the function propagates the result back up the recursive chain, eventually returning the path to the caller.

Alternatively, removing the else block and always returning at the end of the for-loop can also resolve the issue:

for filename in dictionary.keys():
    path = prefix + [filename]
    if not isinstance(dictionary[filename], dict):
        if rqfile in str(os.path.join(*path)):
            return str(os.path.join(*path))
    return get_path(directory[filename], rqfile, path)

By consistently returning the recursive result, the function can now correctly retrieve and return the path to the desired file.

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